Extensometer for indicating amount of elongation of strip material



Aug. 17, 1948. (3. RENDEL ET AL 2,447,26

EXTENSOMETER FOR INDIGATING AMOUNT OF ELONGATION OF STRIP MATERIAL Filed June 28, 1946 gig/V45 Patented Aug. 17, 1948 UNITED STATES PATENT OFFICE EXTENSOMETER FOR INDICATING AMOUNT OF ELONGATION OF STRIP MATERIAL Delaware Application June 28, 1946, Serial No. 679,955

4 Claims. (Cl. 177-311) This invention relates to an extensometer and more particularly to an extensometer for indicating the elongation of strip in rolling mills or other reducing machines. strip having desirable forming characteristics, a certain amount of work must be done on the material as it passes through the rolling mill. The work done elongates the strip and therefore the amount of rolling is usually referred to as percent extension. Since the amount of extension is an important factor in obtaining the desired ductility or forming characteristics of the product, the amount of the extension must be closely controlled. This is particularly true in cold reducing the strip in a temper mill where the percent extension is very low.

Numerous attempts have been made to measure the amount of extension obtained in the moving strip, but none have proved to be entirely successful. Two of the principal drawbacks of the present devices are the inability to indicate the true amount of extension when the amount of elongation is very small and the inability to compensate for variations in speed. The first drawback is particularly noticeable at high strip speed. In some instances a generator is connected to be driven at a speed proportional to that of the entering strip and a second generator is connected to be driven at a speed proportional to that of the rolled strip. While these devices are fairly satis- In order to produce steel factory where heavy drafts (30% in reduction or more) are taken, they may be as much as 200% in error when used in temper rolling where the extension may be between A; to 2%. This error rises from the fact that the reproductibility of a speed measurement made by means of a conventional electric generator may vary as much as plus or minus 1% and this variance is compounded when determining the percent extension. For example, if a 1% extension has been specifled, the exit speed measuring potential would be 1% greater than that of the entry speed measuring potential. Since the potential of each generator may vary 1 of its true value, the reading indicated by the control meter may be 200% in erg-lor in respect to the actual conditions in the m 1.

It is therefore an object of our invention to produce an extensometer which is accurate at all speeds and for all amounts of reduction.

This and other objects will be more apparent after referring to the following specification and attached drawing, in which:

The single figure is a schematic view showing one embodiment of our invention.

Referring more particularly to the drawing, the strip S is shown passing from an uncoiler 2 over an entry deflector roll 4, through a rolling mill 6 and over a delivery deflector roll 8 to the coiler ID on which it is recoiled. By way of example, the operation may be considered as temper rolling in which the strip is given a very small reduction under tension. The strip rotates the deflector rolls at a peripheral speed corresponding to the strip speed at the respective rolls. A motion transmitting device i2 is driven from the entry deflector roll 4 and is electrically connected to a motion receiving device It. The motion transmittin and receiving devices [2 and It may be of any suitable type such as the well known Synchro-tie type. Each of these devices is provided with a three circuit armature winding and a three circuit field winding supplied from a source of alternating current Ll-, L-2 and L-3. Rotation of the transmitter armature produces a synchronous rotating field in the armature of the receiver l4 so that it rotates in synchronism with the armature of transmitter 12. The armature of rece ver i l is connected to the input shaft [6 of a differential gear system I8. The differential gear system [8 has a second input shaft 20 which is suitably driven from the delivery deflector roll 8. The forward motion of the strip revolving shaft 28 produces a forward rotation of the differential gear, while the shaft l6, which is driven from the roll 4 by means of the synchro-tie transmitter i2 and receiver l4, produces a reverse rotation of the differential gear system. It will be evident that the speed of rotation of output shaft 22 of the differential gear system [8 is the algebraic sum of the speeds of the input shafts i6 and 20. Therefore, output shaft 22 rotates at a speed which is directly proportional to the difference between the entry and delivery speeds of the s rip. The shaft 22 is connected to a generator 24. The combination of devices I2, l4 and I8 is the equivalent of a differential synchro-tie system. a differential Selsyn system or .a mechanical d fferential gear system. When rolls 4 and 8 revolve at the same peripheral speed, the output shaft 22 of the difierential gear system l8 does not rotate, but when they revolve at difi'erent speeds, the shaft 22 rotates at a speed which is equal to the difference between the roll speeds. Thus the generator 24 is driven at a speed proportional to the difierence between the speeds of the rolls 4 and 8 and its voltage output is directly proportional to the differential strip speed and serves as a basis for indicating, recording and/or controlling the extension being effect d by th mill. Since only one generator is used in obtainin: the diflerential voltage, the maximum possible err r in the measurement of this difference is 1% (the inherent error oi one speed measuring system).

If the rolling speed is kept constant, the extension being effected by the mill can be obtained by connecting a suitable voltmeter across the-terminals of generator 24 and calibrating the meter to read "percent extension. However, in actual operation. the mill speed varies and the voltage output of generator 24 will vary directly with the mill speed when the percent extension of the strip remains constant. Thus, if the voltage output from generator 24 is 100 millivolts when an extension 01 1% is being obtained with a strip speed of 100 feet per minute, the generator 24 will generate 2000 millivolts or 2 volts when the strip speed is increased to 2000 feet per minute with the extension remaining at 1%. From the foregoing, it is seen that means must be provided for compensating for changes in strip speed in order to obtain an accurate indication of the percent extension at all rates of operating speed. For this purpose we provide a generator 26 which is mechanically driven through transmitter 52 by the entry deflector roll 4 so that the voltage output from generator 26 is directly proportional to the speed of the strip at the roll 3. Since the voltage output of generator 26 varies directly with the diflferential strip speed and the voltage output of generator 26 varies directly with entering strip speed, the ratio between the voltages of generators 24 and it will indicate the percent extension. The ratio of the two voltages can be measured by a modified indicating recording potentiometerv such as a Brown Electronik. The voltage output from generator 26 is connected to a suitable voltage divider network which is comprised of resistors 28 and 30 and slide wire 32. If desired, a capacitor as may be employed to smooth out the voltage ripple caused by the commutator of the armature of generator 28. The voltage output of generator 2-41 is connected to another voltage dividing network which is comprised of resistors 35, 30 and (iii. If desired, a capacitor 62 may be employed to smooth out the voltage ripple caused by the commutator of the armature of generator 26. The contact time it of the slide wire 32 is connected electrically to the vibrating contact 66 of a convertor 68, whose stationary contacts 50 are connected to the primary windings 52 of a center tap transformer 56.

The center tap 56 of the primary windings 52 is connected to a point 53 between resistors 36 and 38. Power for driving the vibrating contact it of convertor it is obtained from an energizing coil which is connected to the lines L-i and L-Z. Generators 2% and 26 have common negative terminals. The voltage drop across slide wire 32 varies directly with the output of generator 26 and the voltage drop across resistor 3t varies directly with the output of generator 26. These two potentials can be balanced by adjusting the position of the sliding contact arm 46 and when the opposed potentials are equal, the position of contact arm 44 represents the ratio between the combined voltages of generators 24 and 26 and the voltage of slide wire 32. Contact arm 44 is connected to an indicating pointer and the slide wire scale is graduated to indicate percent extension. The position of the ,contact arm 46 is adjusted by the convertor 48, a voltage amplifier 66 which is mechanically connected to the contact arm. The convertor 48 is a single-pole. double throw switch operated in synchronism with the power supply line voltage by means oi the energizing coil 60 which is continuously energized from power supply lines L-l and L-2. the switch being polarized by means of a permanent magnet so that one of its stationary contacts 50 will always he in contact with the vibrating contact 40 during the positive half-cycle of the voltage supply and the other stationary contact 50 will be in contact with the vibrating contact 40 during the negative half-cycle. The transformer 54 serves as a coupling device between the measuring circuit, contact arm 44 and point 58. and the electronic voltage amplifier 62. Input transformer 54 has a dual input which, in conjunction with the convertor 48, supplies an alternating current voltage from the secondary winding 00, the magnitude of which is proportional to the direct current unbalance of the measuring circuit and whose phase relation is dependent upon the direct-ion of unbalance. When potential exists between contact arm 44 and point 58, it is impressed on the primary windings 52 of the center tapped input transformer 54 and appearson the secondary winding 68 as an alternating current whose magnitude is proportional to the direct current unbalance. The alternating current from secondary winding 68 is transferred. to the voltage amplifier 62 which amplifies it from a low value of several microvolts to a value of several volts. The voltage output from the voltage amplifier 82 is transmitted to the power amplifier 64, which amplifies the power. Power amplifier 64 controls the balancing motor 66, which is mechanically connected to the contact arm 44. Both the phase relation and the magnitude of the driving power of motor 66 are directly controlled by the voltage amplifier 62. The balancing motor 66 is a reversible induction motor having two windings. One winding is continuously energized from the supply lines L-i and L-ii and the other winding is energized by the power amplifier 84 with a current whose phase relation with respect to that of the line current determines the direction of rotation of the balancing motor 66 and whose magnitude determines the rotational speed oi the balancing motor. If desired, an indicating pointer and a recording pen may be directly connected to the motor 66. When the slide wire contact arm 44 is moved to a point where the opposing potentials are balanced, there is no potential to convert or amplify and the balancing motor stops. The pointers location on the calibrated dial then indicates the percent extension being effected by the mill, and if desired, the recording pen may be used to record the percent extension on a calibrated chart driven by a separate driving motor which is an integral part of the indicating recording potentiometer.

By way of example, it may be assumed that when the strip is moving into the mill at a rate of feet per minute and is being extended 1%, the voltage output from generator 28 will be 1 volt and that of generator 24 will be 100 millivolts. Under these conditions the convertor and amplifiers, through the balancing motor 68, will cause the contact arm of slide wire 44 to balance at a value corresponding to the 1% mark on the scale which is at the point where the opposing voltages are equal. If the mill speed is increased to 2000 feet per-minute the voltage output from generator 26 will increase to 20 volts, thereby increasing the voltage drop between the contact arm 44 and the common netative wire 70. However, at the same time the voltage output from generator 24 increases to 2 volts. Since the voltage output from generators 24 and 26 were each increased by 2000%, the opposed voltages between contact arm 44 and point 55 will remain equal and the slide wire contact arm M will remain at the 1% position on the scale. If the mill is adjusted to effect a 2% extension while operating at 2000 feet per minute, the voltage output from generator 24 will be increased from 2 volts to 4 volts while the voltage output from generator 20 remains at 20 volts. This causes the voltage drop between the negative wire and point 58 to increase in value, which in turn causes the amplifiers to rotate the balancing motor to restore a balance between the two opposing voltages. This will occur when the contact arm 44 reaches the 2% position on the scale.

While one embodiment of our invention has been shown and described it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

We claim:

1. An extensometer for determining the elon gation of strip passing through a reducing machine comprising means connected to be driven at a speed proportional to entering strip speed, means connected to be driven at a speed proportional to exit strip speed, a differential device connected to both of said means for obtaining the difierence in their speeds, a tachometer generator driven by said differential device, a, tachometer generator driven at a speed proportional to entering strip speed, a two-legged bridge having a resistance in one leg and a slide wire in the other leg, said slide wire being connected in series with the second named generator, a movable contact arm for said slide wire, said resistance being connected in series with the first named generator, and means responsive to the difference between the voltage drop across said resistance and the voltage drop across said slide wire for moving said contact arm to balance said voltages, the position of said contact arm when the said voltages are balanced indicating the amount of elongation of the strip.

2. An extensometer for determining the elongation of strip passing through a reducing machine comprising means connected to be driven at a speed proportional to entering strip speed, means connected to be driven at a speed proportional to exit strip speed, a differential device connected to both of said means for obtaining the difference in their speeds, a tachometer generator driven by said diflerential device, a tachometer generator driven at a speed proportional to entering strip speed, a two-legged bridge having a resistance in one leg and a slide wire in the other leg, said slide wire being connected in series with the second named generator, 9. movable contact arm ior said slide wire, said resistance being connected in series with the first named generator, a capacitor in the output circuit or each of the generators, and means responsive to the difference between the voltage drop across said resistance and the voltage drop across said slide wire for moving said contact arm to balance said voltages, the position'of said contact arm when the said voltages are balanced indicating the amount of elongation of the strip.

3. An extensometer for determining the elongation of strip passing through a rolling mill comprising a motion transmitting device connected to be driven at a speed proportional to entering strip speed, a second motion transmitting device connected to be driven at a'speed proportional to exit strip speed, a difierential device connected to said motion transmitting devices for obtaining the difference in their speeds, a tachometer generator driven by said differential device, a tachometer generator driven at a speed proportional to entering strip speed, a two-legged bridge having a resistance in one leg and a, slide wire in the other leg, the slide wire being connected in series with the second named generator, a movable contact for said slide wire,

said resistance being connected in series with the first named generator, and means responsive to the difference between the voltage drop across said resistance and the voltage drop across said slide wire for moving said contact arm to balance said voltages, the position or said contact arm when the said voltages are balanced indicatin the amount of elongation of the strip.

4. An extensometer for determining the elongation of strip passing through a rolling mill comprising a motion transmitting device connected to be driven at a speed proportional to entering strip speed, a second motion transmitting device connected to be driven at a speed proportional to exit strip speed, a differential device connected to said motion transmitting devices for obtaining the difierence in their speeds,

, a tachometer generator driven by said differential device, a tachometer generator driven at a speed proportional to entering strip speed, a two-legged bridge having a resistance in one leg and a slide wire in the other leg, the slide wire being connected in series with the second named generator, a movable contact for said slide wire, said resistance being connected in series with the first named generator, a capacitor in the output circuit of each of the generators, and means responsive to the difierence between the voltage drop across said resistance and the voltage drop across said slide wire for moving said contact arm to balance said voltages, the position of said contact arm when the said voltages are balanced indicating the amount of elongation oi. the strip.

GEORGE H. REN'DEL.

THOMAS F. REED.

narnnences crran The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,039,914 McBane May 5, 1936 2,051,018 Umansky Aug. 11, 1936 2,248,504 Kenny July 8, 1941 2,276,739 Rogers Mar. 17, 1942 

